mkbundle: follow symlinks

some resources are now symlinked to vendor/ so when built with --enable-bundle webif fails to load due to resources not compiled in.

from python os.wak() docs:
```
By default, walk() will not walk down into symbolic links that resolve to directories. Set followlinks to True to visit directories pointed to by symlinks, on systems that support them
```
actual error
```
...
2013-09-24 18:55:32.243 [  ERROR] webui: failed to open src/webui/static/extjs/resources/css/ext-all-notheme.css
2013-09-24 18:55:32.243 [  ERROR] HTTP: 127.0.0.1: /static/extjs/resources/css/ext-all-notheme.css -- 500
2013-09-24 18:55:32.243 [  ERROR] webui: failed to open src/webui/static/extjs/resources/css/xtheme-blue.css
2013-09-24 18:55:32.243 [  ERROR] HTTP: 127.0.0.1: /static/extjs/resources/css/xtheme-blue.css -- 500
...
```